The UK Independence Party has been thrown into disarray after the shock resignation of leader Diane James after just 18 days in charge.
Ms James cited "personal and professional" reasons for her decision, saying that she had not been given sufficient authority to force through changes she wanted to make to the party.
It is also thought that her husband's ill-health and a recent incident in which she was "badly shaken up" after being verbally abused and spat at in a central London street may have played a role in persuading her to step down.
Confusion surrounded her position after she suggested in a statement that she had never formally taken up the role of leader after winning a landslide victory in the race to succeed Nigel Farage on September 16.
Party sources questioned her claim that the election amounted only to a "nomination" as leader which needed to be registered with the Electoral Commission to be formally confirmed.
Ukip's ruling National Executive Committee is expected to hold an emergency meeting within days to choose an interim leader and agree a timetable for the second leadership election within weeks.
Bookmakers installed North-West England MEP Steven Woolfe - who was barred from standing to replace Mr Farage after handing nomination papers in 17 minutes late - as hot favourite, followed by Suzanne Evans, who missed out on the chance to run because of a suspension which has now been lifted.
In her brief time as leader, Ms James had not appointed a deputy, so there is no one in place to act as a stand-in while a new election is arranged. Party officials were unable immediately to say who was leading the party.
Mr Farage poured cold water on speculation he might return to lead the party for a third time, after stints from 2006-09 and 2010-16, telling the Press Association: "Not for 10 million dollars."
And asked if he would take 20 million dollars, he said: "No, I'm not coming back, I'm retired."
Ukip chairman Paul Oakden said on Tuesday: "It is with regret that I have tonight received confirmation that Diane James has chosen to resign as party leader, citing personal and other reasons. I will now look to convene an emergency meeting of our NEC to confirm the process for electing Diane's replacement.
"Whilst the decision is unfortunate, it is one that Diane is entitled to make. We thank her for all her work as leader, and as a hard working MEP, a role she will continue with her customary vigour."
It is understood that Mr Oakden was not informed in advance of Ms James's intention to quit, and received formal confirmation only when she released a statement on Twitter.
In a statement announcing her decision, Ms James said: "It is with great regret that I announce that I will not be formalising my recent nomination to become the new leader of the party with the Electoral Commission.
"Having won the enthusiastic support of party members, I was nominated by them as the new leader at the recent Ukip Bournemouth conference.
"Since that time, I have been in discussion with party officers about the role. It has become clear that I do not have sufficient authority, nor the full support of all my MEP colleagues and party officers to implement changes I believe necessary and upon which I based my campaign.
"For personal and professional reasons, therefore, I will not take the electoral process further."
Ukip insiders questioned suggestions that Ms James had failed to receive support from officials.
The 56-year-old former Conservative councillor, who defected to Ukip in 2011, said she would stay on as an MEP for South-East England.
Elected to the European Parliament in 2014 after achieving national prominence as second-placed candidate in the 2014 Eastleigh by-election, Ms James was the party's first woman leader and won the post by a wide margin after winning the backing of senior Ukip figures including Mr Farage.
But questions were raised about her commitment to the post after she declined to take part in hustings debates around the country with rival candidates.
In September's election, Ms James took 8,451 out of the 17,970 votes cast, romping home ahead of Lisa Duffy - backed by many in the anti-Farage wing of the party - Bill Etheridge, Phillip Broughton and Elizabeth Jones.
In her acceptance speech, she promised to bring a new professionalism to the party, saying: "We are going to confound our critics, we are going to outwit our opponents, we are going to build on our election success that we have achieved to date and do more."
Ukip's only MP, Douglas Carswell, declined to comment on Ms James's resignation, saying in a tweet: "In the middle of supper. Not taking calls about UKIP stuff. It's shepherds pie, by the way."